Synthesis and Characterization of Cobalt Complexes with Radical

Nov 11, 2013 - Haruka Nishiyama , Hideaki Ikeda , Teruhiko Saito , Benjamin Kriegel , Hayato Tsurugi , John Arnold , and Kazushi Mashima. Journal of t...
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Synthesis and Characterization of Cobalt Complexes with Radical Anionic α‑Diimine Ligands Xiao-Juan Yang,*,† Xiaohui Fan,† Yanxia Zhao,† Xuting Wang,† Bin Liu,† Ji-Hu Su,‡ Qingsong Dong,§ Maolin Xu,† and Biao Wu† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, People’s Republic of China ‡ Department of Modern Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China § State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Acacdemy of Sciences, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: Three cobalt complexes, [LCo(μ-Cl)]2·THF· Et2O (1), [LCo(η6-toluene)] (2), and [LCo]2 (3), where L = [(2,6-iPr2C6H3)NC(Me)]2, have been synthesized from reduction of the dichloro cobalt precursor [LCoCl2] by sodium or potassium metal. Single-crystal X-ray diffraction analyses reveal that the complexes display novel structures with the singly reduced form (L•−) of the α-diimine ligand: the dichloro-bridged binuclear complex 1, the mononuclear 2 with an η6-coordinated toluene molecule, and the dimer 3 through Co−aryl interactions of two [LCo] units. The compounds were further characterized by EPR and magnetic studies.

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INTRODUCTION In recent years, metal complexes with low-valent, low-coordinate metal ions have attracted much interest owing to their novel structures (e.g., the metal−metal-bonded compounds) and interesting reactivity toward a variety of small molecules.1,2 A number of low-coordinate mononuclear and binuclear cobalt complexes with a variety of bulky organic ligands, such as the two-coordinate Co(II) complexes CoAr2 (Ar = bulky terphenyl ligands), ArCoN(SiMe3)2,3 and the “masked” two-coordinate Co(II) complex LtBuCo (LtBu = bulky β-diketiminate ligands),4 have been synthesized and their reactivity has been studied.5 On the other hand, noninnocent ligands have been a recent research focus because of their electronic tuning abilities that can result in unusual structures and reactivity. α-Diimine ligands are widely used in both main-group and transition-metal coordination, such as in the late-transition-metal catalysts for olefin polymerization.6 However, cobalt complexes of α-diimine ligands still remain scarce.7 We have been exploring the noninnocence of α-diimine ligands, which, in their reduced (monoanionic or dianionic) forms, proved to be very promising in the stabilization of a number of metal−metal-bonded compounds (e.g., Zn−Zn, Mg−Mg, and Al−Al bonds) and low-valent, low-coordinate mononuclear complexes.1c,2a,8 Moreover, these reduced ligands can also participate directly or indirectly in the reactions with some small molecules. In this current work, we report three low-coordinate and low-valent cobalt complexes supported by an α-diimine ligand, [LCo(μCl)]2·THF·Et2O (1), [LCo(η6-toluene)] (2), and [LCo]2 (3) (L © 2013 American Chemical Society

= [(2,6-iPr 2 C 6 H 3 )NC(Me)] 2 ) (Scheme 1). Complex 1 represents a dichloro-bridged binuclear cobalt complex with αScheme 1. Synthesis of Compounds 1−3

diimines, while the last two compounds feature η6 coordination to a toluene molecule or an aryl ring of the ligand L.

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RESULTS AND DISCUSSION Synthesis. The precursor [LCoCl2] (L = [(2,6-iPr2C6H3)NC(Me)]2) was synthesized by following a literature procedure.8 Complexes 1−3 were obtained from the reaction of [LCoCl2] Received: April 26, 2013 Published: November 11, 2013 6945

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lengths of the NCCN moiety of the α-diimine ligand. When the complex is reduced to the radical anion and further to the dianion, elongation of the C−N bonds and shortening of the C− C bond will be observed. In complex 1, the C−N (1.322(4) Å) bond distances are somewhat elongated, while the C−C (1.435(6) Å) bond is shortened in comparison to the neutral ligand (C−N = 1.280(3), 1.279(3) Å, C−C = 1.494(3) Å). These distances indicate the monoanionic character of the ligands, and thus the Co centers should have the formal oxidation state of +2 in complex 1.1c,8,12,13 This assignment is similar to some related metal complexes of α-diimine ligands with bridging chloride ions.14 Furthermore, EPR studies were carried out to elucidate the electronic structure of complex 1. Since the Co(II) ion adopts a nearly tetrahedral coordination environment, it is likely in the high-spin (SCo = 3/2) state.12a This combines antiferromagnetically with the anion radical to give rise to an effective S = 1 spin state. The EPR measurements indicated that complex 1 was EPR silent or was not thermally accessible to the X-band frequency, which is consistent with the above assignment. [LCo(η6-toluene)] (2). Reduction of [LCoCl2] with 2 equiv of potassium in toluene gave air-sensitive black crystals of the complex [LCo(η6-toluene)] (2; Scheme 1). In this mononuclear complex, the cobalt center is chelated by the two nitrogen donors of a ligand L and η6 coordinated by the arene ring of a toluene molecule (Figure 2). A variety of such η6-arene cobalt complexes

with sodium or potassium metal in diethyl ether or toluene (Scheme 1). These compounds are highly air- and moisturesensitive but are thermally quite stable under argon at room temperature and can be stored for several days without decomposition. Crystal Structures. [LCo(μ-Cl)]2 (1). The reaction of the precursor [LCoCl2] with 2.0 equiv of sodium metal in Et2O afforded the complex [LCo(μ-Cl)]2 (1) as black crystals. The bimetallic structure of 1 (Figure 1) is a rare example of a cobalt

Figure 1. Molecular structure of 1. Thermal ellipsoids are set at the 30% probability level; hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Co···Co = 3.026(1), C(1)−N = 1.322(4), C(1)−C(1A) = 1.435(6), Co−N = 1.939(3), Co−Cl = 2.268(2), Co−Cl(B) = 2.264(2); N−Co−N(A) = 81.8(2), Cl−Co− Cl(B) = 96.2(1), Co−Cl−Co(B) = 83.8(1). Symmetry codes: (A) x, 1 − y, z; (B) 1 − x, y, 1 − z; (C) 1 − x, 1 − y, 1 − z.

complex dimerized by bridging chloride ions and supported by αdiimine ligands, although a few organocobalt complexes bridged by chloride ions have been reported with other ligands such as 2(pyrid-2-yl)quinoxaline and CH(CMeNAr)2.9 The molecule shows a C2h symmetry (space group C2/m). Each cobalt atom is four-coordinate with two N atoms of a ligand and two bridging Cl atoms in a tetrahedral environment (Figure 1), with the CoN2 and CoCl2 planes being strictly perpendicular (dihedral angle 90°) due to the crystallographically imposed symmetry. The Co center sits over the C2N2 moiety with a vertical distance of 0.35 Å, and the dihedral angle between the CoN2 and C2N2 planes is 13.6°. The structure of 1 is similar to those of some binuclear dihalide complexes, such as the cobalt dimer [BrCoC 6 H 3 -2,6Mes2(THF)]2,10 the calcium complex [LCa(μ2-Cl)(THF)2]2,8a and the dibromo and diiodo complexes [{(priso)CoX}2] (priso = [(ArN)2C(N-iPr2)].11 The four equivalent Co−N bond lengths (1.939(3) Å) in 1 are somewhat shorter than those in the related compound [{(priso)CoI}2] (Co−N = 1.989(3) Å) and a dimeric β-diketiminate CoII chloride complex (Co−N = 1.972 Å). The Co−Cl (average 2.266 Å) bond lengths are also shorter than that in the latter compound (Co−Cl = 2.353 Å). In addition, the N−Co−N angle (81.8(2)°) is much smaller than that in the latter complex (N−Co−N = 96.5(1)°), while the Cl− Co−Cl angle is widened by 8.2°.9b The Co···Co separation is 3.026(1) Å in 1. The α-diimine ligands are known to be redox noninnocent, and sometimes the oxidation states of such ligands and the metal ions in their complexes can be ambiguous.12 Some criteria that can be used to determine the oxidation states are the bond

Figure 2. Molecular structure of 2. Thermal ellipsoids are set at the 30% probability level; hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and bond angles (deg): C(1)−N(1) = 1.334(4), C(2)−N(2) = 1.336(4), C(1)−C(2) = 1.414(4), Co−N(1) = 1.980(3), Co−N(2) = 1.982(2), Co−C(toluene) = 2.118(3)−2.193(3) (average 2.163), Co−centroid = 1.642; N(1)−Co−N(2) = 79.7(1).

have been reported, such as the β-diketiminate complex [(HC{C(Me)NC 6 H 3 -2,6-Me 2 } 2 )Co(η 6 -C 7 H 8 )], 5a [3,5iPr2Ar*Co(η6-C7H8)] (Ar* = C6H-2,6-(C6H2-2,4,6-iPr3)2),15 and the amidinato and guanidinato analogues [RC(Ar2NN)]Co(η6-toluene) (R = tBu, NCy2, NiPr2).11 Moreover, the interaction of cobalt with the arene ring has been analyzed. In complex 2, the Co−C distances are in the range 2.118(3)− 2.193(3) Å and the Co−centroid distance is 1.642 Å. These distances are shorter than those in the β-diketiminate analogue (Co−C = 2.207(6)−2.288(5) Å; Co−centroid = 1.747(2) Å)7a and [RC(Ar2NN)]Co(η6-toluene) (1.659, 1.662, 1.668 Å) but are comparable with those in some other related complexes (e.g., [3,5-iPr2Ar*Co(η6-C7H8)]; Co−centroid = 1.659(1) Å). In the molecule of 2, the Co atom is located on the C2N2 plane, and the N-aryl rings of ligand L are almost perpendicular to the five-membered CoC2N2 cycle (dihedral angles 86.1 and 85.3°). The arene ring of the coordinated toluene is also perpendicular to the CoC2N2 cycle (dihedral angle 88.4°). The cobalt atom 6946

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resides in such an environment that the Co−centroid connection line is almost collinear with the axis of the ligand L (with the angle of the center of C1C2−Co−centroid being 178.9°). This is similar to the case for the complex [3,5-iPr2Ar*Co(η6-C7H8)], which has a Cipso−Co−centroid angle (167.6°) that is close to linearity.15 The Co−N bond lengths in 2 (1.980(3) and 1.982(2) Å) are somewhat longer than those in 1 (Co−N = 1.939(3) Å) but are comparable to other reported cobalt analogues (Co−N = 2.013 Å (average)). The N−Co−N angle (79.7(1)°) is slightly smaller than that in 1 (81.8(2)°) and is much smaller than in analogous α-diimine cobalt complexes (91.3(2)°). The C−N (1.334(4) and 1.336(4) Å) and C(1)−C(2) (1.414(4) Å) distances in 2 are consistent with the monoanionic radical form of the ligand (1−). The EPR spectrum of compound 2 at room temperature (Figure S1, Supporting Information) shows the expected multiline paramagnetic signals (centered at g = 2.073) consistent with the radical state of L, as reported for α-diimine ligands.1c,8,12,13 The low-temperature EPR spectrum (at 77 K in THF; Figure S2, Supporting Information) also shows the signal of the ligand radical, but the isotropic hyperfine splitting spectral pattern seen at room temperature is replaced by a broad signal due to the anisotropic g value and hyperfine coupling, as well as the spin−spin interaction between the radicals in the frozen solution. Correspondingly, the formal oxidation state of cobalt is +1. The solid-state magnetic studies of 2 indicated that the effective magnetic moment (μeff) at room temperature is 3.25 μB (Figure S4, Supporting Information), which is close to the calculated value of 3.31 μB corresponding to the contributions from high-spin (S = 1) Co(I) and the radical ligand (S = 1/2). This magnetic moment is comparable with the result of a reported CoL3 complex with a similar electronic structure, in which the cobalt center shows a formal oxidation state of +1 and one of the three α-diimine ligands is in the radical anionic form.12a The temperature-dependent magnetic behavior of complex 2 will be discussed in detail below (together with that of complex 3). The existence of high-spin Co(I) is similar to the case for analogous compounds with Co−arene interactions: [(HC{C(Me)NC6H3-2,6-Me2}2)Co(η6-C7H8)] (2.7 μB) and (3,5-iPr2-Ar*)Co(η6-C7H8) (3.37 μB).5a,15 The high-spin d8 Co(I) configuration was further confirmed by the plots of χM−1 versus T and μeff versus T (Figure S2, Supporting Information). [LCo]2 (3). Complex 3 was obtained by the reduction of [LCoCl2] with Na in Et2O/cyclohexane (7/3 v/v). Alternatively, it can also be prepared from the reaction of L, potassium metal, and CoCl2. Single crystals for X-ray diffraction studies were grown from THF. The complex shows a centrosymmetric dimeric structure (Figure 3), in which each cobalt atom is coordinated by the two nitrogen atoms of an α-diimine ligand and is η6-bonded to an aryl ring of the ligand attached to the other metal center. The coordination environment of the cobalt atom is very close to that of the mononuclear complex 2, with Co− C(aryl) distances in the range 2.167(3)−2.230(3) Å and a Co− centroid distance of 1.675(1) Å. In addition, this dimerized structure is somewhat similar to that for the iron and cobalt derivatives [Ar′M]2 (Ar′ = C6H3-2,6(2,6-iPr2C6H3)2), in which the iron and cobalt atoms are η1bonded to a terphenyl ligand and η6-bonded to a flanking ring of the other ligand.16 However, there are significant differences between the two cases. First, as in 2, the cobalt−centroid connection line is also nearly collinear with the axis of the ligand L (the angle of the center(C1C2)−Co−centroid(aryl) is 179.0°). In contrast, in [Ar′Co]2 the Cipso−Co−centroid angle

Figure 3. Molecular structure of 3. Thermal ellipsoids are set at the 30% probability level; hydrogen atoms and isopropyl groups of L have been omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Co−N(1) = 2.020(2), Co−N(2) = 1.991(2), N(1)−C(1) = 1.337(3), N(2)−C(2) = 1.329(4), C(1)−C(2) = 1.413(4), Co−C(aryl) = 2.167(3)−2.230(3) (average 2.190), Co−centroid = 1.675(1), Co···Co = 4.062(1); N(1)−Co−N(2) = 79.3(1). Symmetry code: (A) 1 − x, −y, 1 − z.

is 143.7° (a bending of 36.3° from linearity).16 This difference leads to a Co···Co separation (4.062(1) Å in 3) remarkably longer than that (2.803(1) Å) in [Ar′Co]2. Second (and also as a consequence of the “linear” interactions), the Co to aryl ring interaction is stronger in complex 3 than in [Ar′Co]2, with the Co−centroid distance being shorter by about 0.09 Å (1.675(1) vs 1.764(2) Å). Notably, these distances are much shorter than those in the structurally analogous dichromium compound [Ar′CrCrAr′] (Cr−centroid = 2.203(6) Å), which has 5-fold bonding and a very short Cr−Cr distance (1.8351(4) Å) between the two Cr(I) centers.17 From these examples it can be seen that with the weakening of the metal−arene (or aryl) interactions, the metal−metal distance may be shortened to facilitate metal− metal bonding. The five-membered CoN2C2 chelating ring is nearly planar, in which the Co atom resides about 0.015 Å out of the C2N2 plane, and the C2N2 planes of the two ligands are coplanar. The Co−N distances (2.020(2) and 1.991(2) Å) are longer than the average Co−N bond lengths in 1 and 2, which should be a result of both the steric restraints and electronic effects. The N−Co−N angle (79.3(1)°) in complex 3 is somewhat smaller than that in 1 (81.8(2)°) but is identical with that in 2 (79.7(1)°). Similar to the case for complex 2, the average C−N (1.333 Å) and C−C (1.413(4) Å) bond lengths of the C2N2 moiety also demonstrate the monoanionic radical form of the ligand, which is accompanied by the Co(II) in the precursor [LCoCl2] being reduced to Co(I). The room-temperature EPR spectrum of 3 shows hyperfine signals with a g value of 2.008 (Figure S3, Supporting Information), suggesting the existence of α-diimine ligands in a radical monoanionic form as in complex 2.1c,8,13 The magnetic measurements of complex 3 in the solid state revealed μeff = 3.30 μB (χMT = 1.362 cm3 K mol−1) per monomer at room temperature, which is close to the value for the mononuclear analogue 2 (μeff = 3.25 μB) and is excellently consistent with the calculated value of 3.31 μB for two noninteracting high-spin d8 electronic configuration Co(I) ions (S = 1) together with the radical monoanionic ligands. The magnetic data of 3 are similar to those for the complexes LtBuCo(THF)4 and (3,5-iPr2Ar*)Co(η6-C7H8) (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2).15 However, the related dimeric compound [Ar′Co]2, which also features 6947

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by sodium/benzophenone and distilled under argon prior to use. Anhydrous CoCl2 was purchased from Alfa Aesar. The ligand (L)19 and complex [LCoCl2]7 were prepared according to literature procedures. EPR spectra were recorded on a Bruker EMX-10/12 spectrometer. Variable-temperature magnetic susceptibility data from 1.9 to 300 K were obtained on polycrystalline samples using a Quantum Design MPMS-XL7 SQUID magnetometer. Synthesis of [CoL(μ-Cl)]2·THF·Et2O (1). Potassium (0.04 g, 1.0 mmol) was added to a solution of the precursor [LCoCl2] (0.548 g, 1.0 mmol) in diethyl ether (50 mL). The mixture was stirred for 3 days. It was then filtered and the filtrate concentrated to about 30 mL. Slow evaporation of the filtrate at ca. −20 °C for several days afforded the product as black crystals. Crystal yield: 0.60 g, 56%. Anal. Calcd for C56H80Cl2Co2N4·C4H8O (1070.10): C, 67.34; H, 8.29; N, 5.24. Found: C, 67.22; H, 8.27; N, 5.22. Synthesis of [(LCo(η6-toluene)] (2). Complex 2 was synthesized by a method similar to that employed for 1, from potassium (0.08 g, 2.0 mmol) and [LCoCl2] (0.548 g, 1.0 mmol) in toluene (50 mL). Crystal yield: 0.30 g, 54%. Anal. Calcd for C35H48CoN2 (555.68): C, 75.65; H, 8.71; N, 5.04. Found: C, 75.36; H, 8.69; N, 5.03. EPR (toluene, room temperature): g = 2.073. μeff = 3.25 μB at 298 K. Synthesis of [LCo]2 (3). Complex 3 was synthesized by a method similar to that of 1, except that sodium (0.046 g, 2.0 mmol) was used to reduce [LCoCl2] (0.548 g, 1.0 mmol) in diethyl ether (50 mL). Crystal yield: 0.33 g, 35%. Anal. Calcd for C56H80Co2N4 (927.10): C, 72.55; H, 8.70; N, 6.04. Found: C, 72.97; H, 8.71; N, 5.95. EPR (Et2O, room temperature): g = 2.008. μeff = 3.30 μB at 298 K (per monomer). X-ray Crystal Structure Determination. Diffraction data for complexes 1−3 were collected on a Bruker SMART APEX II diffractometer at 153 K with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). An empirical absorption correction using SADABS was applied for all data.20 The structures were solved by direct methods using the SHELXS program. All non-hydrogen atoms were refined anisotropically by full-matrix least squares on F2 by the use of the SHELXL program.21 Hydrogen atoms bonded to carbon were included in idealized geometric positions with thermal parameters equivalent to 1.2 times those of the atom to which they were attached. One disordered Et2O molecule in complex 1 was treated with the SQUEEZE command. The crystal data and structure refinement details of complexes 1−3 are given in Table S1 (Supporting Information).

Co−arene interactions but has a bent coordination geometry as mentioned above, shows the low-spin Co(I) d8 configuration.16 For complexes 2 and 3, the effective magnetic moment (μeff) in the temperature range 1.9−300 K increases slowly to about 3.30 μB (at room temperature) (Figures S4 and S6, Supporting Information), and thus the paramagnetic centers (Co(I), SCo = 1; α-diimine chelate ligand radical L•−, SL = 1/2) are expected to have antiferromagnetic coupling. Therefore, modeling of the magnetic susceptibility data of 2 and 3 was carried out in the form of χMT−T (Figure 4 and Figure S5; see the Supporting

Figure 4. Temperature-dependent magnetic susceptibilities of 3 recorded at 2 kOe. The solid line is the optimized fitting (SL = 1/2 and SCo = 1) with parameters g = 2.12, J1 = −38.34 cm−1, and θ = −6.72 (gCo = ge) (R = 2.7 × 10−4; see the Supporting Information for more details).

Information for details), and negative coupling constants (J = −43.3 cm−1 for 2 and −38.3 cm−1 for 3, respectively) were obtained. The results demonstrate that the metal center is antiferromagnetically coupled with the ligand radical. However, the coupling constants are smaller than that for the cobalt(II) complex [(L•−)2Co] (J = −504 cm−1).18a The magnetic properties of 2 and 3 are very similar to the those for the aforementioned related Co(I) complex containing a radical anionic α-diimine ligand, which was assigned high-spin d8 Co, being antiferromagnetically coupled with the organic radical.12a However, the behavior is different from that for the low-spin Co(I) complexes (PDI)CoN2 with a bis(imino)pyridine radical anion (SPDI = 1/2).18b

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Figures and CIF files giving EPR spectra, magnetic susceptibilities, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION The synthesis and structures of the three low-coordinate Co(I) or Co(II) complexes [LCo(μ2-Cl)]2·THF·Et2O (1), [LCo(η6toluene)] (2), and [LCo]2 (3) are reported. Complex 1 is a dichloro-bridged Co(II) α-diimine complex, while 2 and 3 feature the η6 coordination of the Co(I) center to the arene ring of either a toluene molecule or another ligand (through dimerization in the latter case). In all of the complexes, the noninnocent α-diimine ligand L has been reduced to its radical monoanionic form (L•−), which (together with the doubly reduced dianion) not only can effectively stabilize low-valent metal complexes but may also lead to a rich variety of novel structures. EPR and magnetic studies further confirmed the electronic structures of the complexes.

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ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Author

*E-mail for X.-J.Y.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21273170). REFERENCES

(1) (a) Resa, I.; Carmona, E.; Gutierrez-Puebla, E. Science 2004, 305, 1136. (b) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.; Gutierrez-Puebla, E.; Monge, A.; Galindo, A.; del Río, D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693. (c) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F.; Liang, Y. M.; Wu, B. Chem. Commun. 2007, 2363. (d) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754. (2) (a) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Wu, B. J. Am. Chem. Soc. 2009, 131, 4210. (b) Fohlmeister, L.; Liu, S. S.; Schulten, C.; Moubaraki, B.; Stasch, A.; Cashion, J. D.; Murray, K. S.; Gagliardi, L.; Jones, C.

EXPERIMENTAL SECTION

General Methods. All manipulations were carried out by using Schlenk techniques under an atmosphere of N2 or drybox techniques. Solvents (tetrahydrofuran, diethyl ether, and cyclohexane) were dried 6948

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Organometallics

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dx.doi.org/10.1021/om4003686 | Organometallics 2013, 32, 6945−6949